1. Field of the Invention
The invention relates to the field of substrate oxidation and more specifically to a method for in situ moisture generation and a rapid thermal steam oxidation process.
2. Description of Related Art
In the fabrication of modern integrated circuits, such as microprocessors and memories, oxidation processes are used to passivate or oxidize semiconductor films. Popular methods to oxidize silicon surfaces in film such as polysilicon gate electrodes and substrates include pure oxygen (O.sub.2) and a water vapor or steam (H.sub.2 O) oxidation process. In either case, the oxygen or water vapor is brought into an oxidation chamber to react with the silicon surfaces to form silicon dioxide (SiO.sub.2).
Present steam oxidation processes generally take place in multi-wafer, resistively-heated "hot wall" furnaces. These steam oxidation processes typically use a pyrogenic torch or bubbler located outside of the reaction chamber in which the steam oxidation process is to take place. In the case of a pyrogenic torch, a hydrogen-containing gas and an oxygen-containing gas are ignited by flame in a reaction area at atmospheric pressure and located away from and generally in a different chamber than the chamber in which wafers are placed. The flame ignition occurs at atmospheric pressure. A problem associated with pyrogenic torch methods is that, for safety reasons, only certain concentration ratios of hydrogen-containing gas and oxygen-containing gas can be utilized. Limiting the available gas ratio unduly restricts the ability to generate ambients with desired concentrations of H.sub.2 O/H.sub.2 or H.sub.2 O/O.sub.2. For example, in order to keep a stable flame burning, torch methods typically require H.sub.2 :O.sub.2 ratios of more than 0.5:1 and less than 1.8:1, respectively. Bubblers are also undesirable for moisture generation in that they can be a significant source of contamination and because they cannot accurately and reliably control the amount of moisture generated.
Another problem associated with the use of pyrogenic torches and bubblers is that these methods are not easily implemented into modern rapid thermal heating apparatus which utilize light sources for rapid temperature ramps and reaction times measured in terms of seconds as opposed to minutes and hours. Rapid thermal heaters are preferred over resistively heated furnaces because of their excellent temperature uniformity and control which provides for more uniform processing and because their short reaction times reduce the thermal budget of fabricated devices.
In many oxidation processes for ultra-high performance integrated circuit applications, a pure SiO.sub.2 film is not desirable as the final structure. For example, although a SiO.sub.2 film may provide adequate insulative properties, thin SiO.sub.2 films have been found to be penetrable by dopants leading to undesirable results. For example, in complementary metal oxide semiconductor (CMOS) circuits, gate doping is utilized, in part, to lower the threshold voltage (V.sub.T) associated with an individual transistor device. Thus, for example, a polysilicon gate will be doped with boron as part of a PMOS device or phosphorus, arsenic, or antimony for an NMOS device. As the gate oxide beneath the polysilicon gate gets smaller, for example in the range of 0.1-0.20 microns, dopants infused into the gate, particularly boron, diffuse through the gate oxide, particularly during a high temperature annealing activation step to activate the dopants in the gate and the diffusion regions. In the case of boron, the boron diffuses through the gate oxide and gets deposited in the channel beneath the transistor device adding more charge. The additional charge becomes scattering centers to charge carriers conducting the current. The scattering creates electric field changes which degrade the mobility of the device. The diffusion of the boron into the channel also unacceptably shifts the V.sub.T.
To prevent dopants from diffusing through thin oxides, such as boron through a thin gate oxide, prior art processes have added to the ambient nitrogen-containing sources such as nitrous oxide (N.sub.2 O), nitrogen oxide (NO), and ammonia (NH.sub.3), each typically having a nitrogen content of zero to five percent or more. The nitrogen-containing material forms a film or layer (typically a silicon nitride (Si.sub.3 N.sub.4) or silicon oxynitride (Si.sub.x N.sub.y O.sub.z) film or layer) that acts as a barrier layer to prevent the diffusion of dopants through the oxide.
In the case of gate oxides, prior art methods place nitrogen-containing materials films at the gate oxide-silicon interface. This has deleterious effects on device performance, by creating scattering centers and degrading channel mobility. To date, however, efforts to place an oxide (SiO.sub.2) at the semiconductor surface and retain the nitrogen-containing material have proved costly in terms of the time involved (e.g., oxidation rate) and the thermal budget associated with the oxidation. What is needed is a method and apparatus for reoxidizing a surface of a semiconductor with material having a nitrogen-containing material in an efficient manner in terms of oxidation rate and thermal budget.